The past few decades have seen many shifts in electronics and semiconductor packaging that have impacted the entire semiconductor industry. The introduction of surface-mount technology (SMT) and ball grid array (BGA) packages were generally important steps for high-throughput assembly of a wide variety of integrated circuit (IC) devices, while, at the same time, allowing reduction of the pad pitch on the printed circuit board. Conventionally packaged ICs have a structure basically interconnected by fine gold wire between metal pads on the die and electrodes spreading out of molded resin packages. Dual Inline Package (DIP) or Quad Flat Package (QFP) are fundamental structures of current IC packaging. However, increased pin count peripherally designed and arranged around the package typically results in too short of a pitch of lead wire, yielding limitations in board mounting of the packaged chip.
Chip-scale or chip-size packaging (CSP) and BGA packages are just some of the solutions that enable dense electrode arrangement without greatly increasing the package size. CSP provides for wafer packaging on a chip-size scale. CSP typically results in packages within 1.2 times the die size, which greatly reduces the potential size of devices made with the CSP material. Although these advances have allowed for miniaturization in electronic devices, the ever-demanding trend toward even smaller, lighter, and thinner consumer products have prompted even further attempts at package miniaturization.
To fulfill market demands toward increased miniaturization and functionality, WLCSP has been introduced in recent years for generally increasing density, performance, and cost-effectiveness, while decreasing the weight and size of the devices in the electronic packaging industry. In WLCSP, the packaging is typically generated directly on the die with contacts provided by BGA packages and bump electrodes. Recent advanced electronic devices, such as mobile phones, mobile computers, camcorders, personal digital assistants (PDAs), and the like, utilize compact, light, thin, and very densely packaged ICs. Using WLCSP for packaging smaller die size devices with lower numbers of pins, corresponding to larger numbers of chips on one wafer, is, therefore, usually advantageous and cost-effective.
One disadvantage of current WLCSP technology is the formation of cracks between the solder ball and the electrode post. The solder ball or bump is typically placed onto the bump electrode or post directly, relying on the soldered joint for structural integrity. The different layers making up the WLCSP device typically have different coefficients of thermal expansion (CTEs). As a result, a relatively large stress derived from this difference is exhibited on the joint between the post and the bump electrode, which often causes cracks to form in the bonding area between the bump electrode/post and the solder ball or bump.
FIG. 1 is a cross-section of a typical, single solder ball of WLCSP feature 10. WLCSP feature 10 is formed directly on die 100. Copper pad 102 is formed on die 100. Copper pad 102 acts as a contact and bonding pad for solder ball 101. During the soldering process, intermetallic compounds (IMC) are naturally formed in a layer, i.e., IMC formation layer 103, at the joint between solder ball 101 and copper pad 102. While existence of IMC formation layer 103 generally signifies a good weld between the solder and the substrate, it is usually the most brittle part of the weld. Because the weld joint is so small in WLCSP, cracks, such as crack 104, may form more easily under the stresses experienced at the joint, and such cracks, because of the size of the overall package, may be more damaging. A small crack that starts along one side of solder ball 101, such as crack 104, may easily propagate across the length of the straight solder joint.
One method that has been suggested to diminish this stress cracking is described in U.S. Pat. No. 6,600,234, to Kuwabara, et al., entitled, “MOUNTING STRUCTURE HAVING COLUMNAR ELECTRODES AND A SEALING FILM.” This method describes forming a sealing film using multiple layers where a portion of the bump electrode protrudes from the sealing film. The protruding electrode assists in absorbing part of the stress caused by the difference in the CTE. The multiple layers of the sealing film are also selected to have graduated CTEs, such that the CTE of the film near the substrate is close to the CTE of the substrate, while the CTE of the film near the circuit substrate is close to the CTE of the circuit substrate. This graduated CTE helps alleviate the stresses that would be caused by sharply different CTEs. However, the multiple layers of the sealing film still usually exhibit a weak shear strength and do not reduce the propagation of any cracks that may form in the IMC layer, thus, reducing the overall reliability of the joint.